(This exercise might be repeated with the more easily interpreted Sample
Images / Filter Samples / face.tiff, which does not show the same illusion,
or with the Sample Images / Illusions / Mountains.tiff, where the
illusion is so strong that the image is difficult to interpret when inverted.

Note the 'hole' in the center.

Try inverting the pixel values (this is not the operation to give the
illusion, but might be done by mistake, so do it anyway) using the Edit
-> Invert menu.

The 'hole', might still appear to be a hole, although this 'negative'
image is difficult to interpret.

Invoke the Edit -> Invert menu again to restore the image to its
original form. (Or use the File -> Revert to Saved menu or Edit
/ Undo).